Crystal filter devices are commonly used as an intermediate frequency (IF) filter for mobile cellular telephone applications. Conventionally crystal filters are commercially available as two pole filters housed in a single package having a standard outline. Where greater attenuation is required four pole filters may be used. Four pole filters can be implemented using a matched pair of two pole filters in combination. In this context "a matched pair" refers to a pair of devices intended for use together in the same circuit.
Traditionally, matched pairs of crystal filter devices are individually mounted by hand onto a printed circuit board. The assembly operative is able to check that the two devices about to be mounted on a particular circuit board are intended to be used as partners.
Although in theory automated assembly is possible special problems arise with matched pairs of devices. For example on an automated assembly line, if the two devices of the matched pair are due to be mounted successively and one of the devices is misaligned, or even missing altogether, the fault is unlikely to be detected until after the other device has been mounted on the circuit board. In this case the circuit board is effectively rendered useless because it is not viable subsequently to locate and individually assemble the missing or mis-aligned device or to provide an alternative device because of the difficulty of matching. Also, if the devices are provided in a continuous sequence, for example on a reel of embossed tape, as is common for devices which are to be placed individually on an automated assembly line, there is a risk that synchronization may be lost whereby one device of a matched pair is assembled in combination with a device from the next matched pair of devices. Once such a loss of synchronization has occurred many circuit assemblies may be manufactured with mis-matched pairs of crystal filters. The cost of omitting or mis-aligning a device placement, or the placement of a mis-matched pair can be very significant indeed.